This will/should do it:
_Flux Menu - Preferences.kmmacros (2.3 KB)
###Notes:
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The whole stuff with “timeout” and “killall System Events” was necessary because of a strange delay caused by the first AXPress. I didn’t find a better way to work around it. Credits to Aviral Bansal for the ‘killall’ idea.
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In case you modify the script, attention with the ellipsis in “Preferences...”: these are three separate dots, not the ellipsis character.
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If it doesn’t work set the timeout to 0.2 or 0.3, etc.
Edit (2017-01-19):
I have posted a more complete standalone version of the script over at the f.lux forum.